Mismatches between engine delivered power and propeller absorbed power will cause unacceptable variations in propeller speed. Mismatches generally occur when the propeller governor is unable to change the speed governing control fast enough to follow changes in scheduled power. For example, ground mode operation schedules propeller pitch to achieve a scheduled power. These changes in propeller pitch can change propeller absorbed power faster than the engine can change delivered power, causing a power mismatch which will yield unacceptable variations in propeller speed.
It is important to keep the speed of the propeller nearly constant during ground operations. This is necessary because of the use of auxiliary equipment, such as alternators, being driven from the propeller gear box. Auxiliary equipment requires that the change in propeller speed not be great. It is also known in the art that when propeller power is lowered, the propeller absorbs less power quicker than the engine can decrease power, and therefore propeller overspeed conditions result. Stated another way, if a decrease in blade angle is commanded, the propeller will accelerate and propeller overspeed may result because of the lag in the engine. Ideally, the goal during ground operations is a constant propeller speed.
The present practice is to partially address the problem of control of propeller speed by including a propeller pitch rate limit to limit the rate of change of propeller absorbed power. This is just a fix.RTM.d rate limit on propeller angle rate of change.
In Ground Mode operation where engine fuel is used to govern propeller speed, and propeller pitch is scheduled as a function of commanded power (PLA), a rapid change in commanded PLA will cause a rapid change in propeller pitch with a corresponding rapid change in propeller absorbed power. The present art partially addresses this problem by including a propeller pitch rate limit to limit the rate of change of propeller absorbed power. However, there still are situations when engine constraints prevent changes in engine delivered power fast enough to match the scheduled changes in propeller absorbed power. The result of this power mismatch is an undesirable excessive change in propeller speed.